Methods and systems of holographic interferometry

ABSTRACT

A holographic interferometer, comprising: at least one imaging device capturing an interference pattern created by at least two light beams; and at least one aperture located in an optical path of at least one light beam of the at least two light beams; wherein the at least one aperture is located away from an axis of the at least one light beam, thus transmitting a subset of the at least one light beam collected at an angle range.

RELATED APPLICATIONS

This application is a National Phase of PCT Patent Application No.PCT/IL2018/050617 having International filing date of Jun. 6, 2018,which is a Continuation-in-Part (CIP) of U.S. patent application Ser.No. 15/614,687 filed on Jun. 6, 2017. The contents of the aboveapplications are all incorporated by reference as if fully set forthherein in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates toholographic imaging and, more particularly, but not exclusively, tomethods and systems of three dimensional measurements using holographicinterferometry.

Holographic imaging, which records amplitude and phase information oflight arriving from an object (such as integrated circuit (IC)semiconductor wafers or flat panel display (FPD)), may be used inmicroscopy to reconstruct the 3D profile of the object, i.e. therelative height of each point in the image.

Some methods use zero angle between the object and reference image, anda phase scanning mechanism, which, combined with multiple imageacquisitions at the same object location, give the possibility toseparate between the phase and amplitude information. These methods mayeven use illumination with a very short coherent length. However, suchschemes are generally too slow for applications in which the objectneeds to be laterally scanned in limited time, such as wafer inspection.

In order to achieve fast lateral scanning of the object usingholographic interferometry, it is desired to be able to extract thephase information from a single image. This may be done by introducing anon-zero angle between the object beam and the reference beam, and theuse of coherent illumination. The spatial frequency in the image dependson the angle between the object imaging optical axis and the referenceimaging optical axis. When the object has for example a raised surface,the interference lines shift. By analyzing the images, it is possible toextract the phase change of the interference lines, and from that deducethe height of the features in the object.

To be able to extract the phase information from the image, theinterference lines need to be with a density low enough so that thecamera pixelization will not average them out, but high enough to have agood lateral resolution of the phase information (this resolution istypically one cycle of interference lines).

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the present inventionthere is provided a holographic interferometer, comprising: at least oneimaging device capturing an interference pattern created by at least twolight beams; and at least one aperture located in an optical path of atleast one light beam of the at least two light beams; wherein the atleast one aperture is located away from an axis of the at least onelight beam, thus transmitting a subset of the at least one light beamcollected at an angle range. This separates the beams arriving fromdifferent chief angles of collected light.

Optionally, the at least one aperture includes at least two apertures,each differently located away from an axis of a respective of the atleast two light beams than another of the at least two apertures, thustransmitting a substantially different subset of the respective of theat least two light beams.

More optionally, the holographic interferometer further comprises atleast two imaging devices, each capturing an interference patterncreated by light passing through one of the at least two apertures.

More optionally, the holographic interferometer, simulating differentwavelengths from a single wavelength light source, further comprises: amonochromatic coherent light source; and an optical setup which splitslight from the light source into a first light beam which illuminates asubject structure and a second light beam which illuminates a referencemirror; and combines the first light beam and the second light beam to asingle combined beam. Using different effective wavelength from a singlewavelength light source thus removes the need for an expensive, multiplewavelength light source.

More optionally, an optical path of the first light beam is opticallyidentical to an optical path of the second light beam.

More optionally, the holographic interferometer has no reference mirror,and each of the at least two apertures is located in an optical path ofone of the at least two light beams.

Optionally, the holographic interferometer further comprises: at leastone beam splitter which splits an original light beam into the at leasttwo light beams.

More optionally, the holographic interferometer has no reference mirror,and the beam splitter splits a beam reflected from a subject structureinto the at least two light beams.

More optionally, the holographic interferometer further comprisesmonochromatic coherent light source illuminating the subject structure.

More optionally, the subject structure is illuminated by ambient light.

More optionally, the subject structure is a diffusive-reflecting object.

More optionally, the holographic interferometer further comprises aspectral filter.

More optionally, the at least one beam splitter includes at least onepolarized beam splitter.

More optionally, the at least one polarized beam splitter splits theoriginal beam into the at least two light beams having two orthogonalpolarizations.

More optionally, the holographic interferometer further comprises ahalf-wavelength waveplate.

Optionally, at least one of position, size and shape of the at least oneaperture is dynamically controlled to create multiple differentinterference patterns.

Optionally, a magnification of the holographic interferometer isdynamically controlled to create multiple different interferencepatterns.

Optionally, a focus of the holographic interferometer is dynamicallycontrolled to create multiple different interference patterns.

According to an aspect of some embodiments of the present inventionthere is provided a holographic interferometer, comprising: at least onebeam splitter which splits an original light beam into at least twolight beams; and at least one imaging device capturing an interferencepattern created by the at least two light beams after reflection from anobject; wherein the at least two light beams have different angle ofincident on the object.

Optionally, the holographic interferometer further comprises at leastone mirror which changes the distance of at least one light beam of theat least two light beams from an original optical axis of the originallight beam to create the different angle of incident.

Optionally, the at least one beam splitter includes at least onepolarized beam splitter.

Optionally, the at least one imaging device includes at least twoimaging devices, each capturing an interference pattern created by lighthaving different polarization angle.

According to an aspect of some embodiments of the present inventionthere is provided a method of setting a holographic interferometer,comprising: positioning at least one imaging device capturing aninterference pattern created by at least two light beams; andpositioning at least one aperture located in an optical path of at leastone light beam of the at least two light beams; wherein the at least oneaperture is located away from an axis of the at least one light beam,thus transmitting a subset of an the at least one light beam collectedat an angle range.

According to an aspect of some embodiments of the present inventionthere is provided a holographic interferometer, comprising: at least onemonochromatic coherent light source; an imaging device which captures aninterference pattern; and an optical setup which splits light from thelight source into a first light beam which illuminates a subjectstructure and is reflected into the imaging device; and a second lightbeam which illuminates a reference mirror and is reflected into theimaging device to create the interference pattern; wherein an opticalpath of the first light beam is optically identical to an optical pathof the second light beam. This ensures that the phase difference betweenthe object image and the reference image is essentially determined bythe object features and the planned angle between the images.

According to an aspect of some embodiments of the present inventionthere is provided a method of obtaining height measurement of a subjectstructure from an image produced by a holographic interferometer,comprising: receiving a digital image of an interference pattern from animaging device of a holographic interferometer, the interference patternis created by an imaging of a subject structure; selecting atwo-dimensional section of the image; analyzing the interference patternin the two-dimensional section to calculate phase of the interferencepattern; and estimating height of a respective section of the subjectstructure from the phase. This limits the spatial smearing and iscomputationally cheaper than common methods.

Optionally, size of the two-dimensional section is smaller than tentimes a theoretical optical spot size of the holographic interferometer.

Optionally, size of the two-dimensional section is smaller than a tenthof a size of the image.

Optionally, the interference pattern includes the sum of interferenceimages of at least two wavelengths.

Optionally, the analyzing includes Fourier transform over thetwo-dimensional section.

Optionally, the interference pattern has substantially integer number ofcycles inside the two-dimensional section.

Optionally, the analyzing includes iterative calculation of an amplitudeand the phase.

According to some embodiments of the invention there is provided acomputer readable medium comprising computer executable instructionsadapted to perform the method.

According to an aspect of some embodiments of the present inventionthere is provided a software program product for obtaining heightmeasurement of a subject structure from an image produced by aholographic interferometer, comprising: a non-transitory computerreadable storage medium; first program instructions for receiving adigital image of an interference pattern from an imaging device of aholographic interferometer, the interference pattern is created by animaging of a subject structure; second program instructions forselecting a two-dimensional section of the image; third programinstructions for analyzing the interference pattern in thetwo-dimensional section to calculate phase of the interference pattern;and fourth program instructions for estimating height of a respectivesection of the subject structure from the phase; wherein the first,second, third, and fourth program instructions are executed by at leastone computerized processor from the non-transitory computer readablestorage medium.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein may be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention may involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1 is a schematic illustration of an exemplary twin optical pathsholographic imaging setup, according to some embodiments of the presentinvention;

FIG. 2 is a flowchart schematically representing a method of obtainingheight measurement of a subject structure from an image produced by aholographic interferometer, according to some embodiments of the presentinvention;

FIG. 3 is an exemplary super pixel from an image expected as a responseto different feature z-height of the subject structure, according tosome embodiments of the present invention;

FIG. 4 is a schematic illustration of an optical setup of a holographicinterferometer for taking multiple interferometric images with a singlewavelength, according to some embodiments of the present invention;

FIG. 5 is a schematic illustration of an aperture plane, according tosome embodiments of the present invention;

FIG. 6 is a schematic illustration of an alternative optical setup of aholographic interferometer for taking multiple interferometric imageswith a single wavelength, according to some embodiments of the presentinvention;

FIG. 7 is a simplified example that compares between the optical pathlength difference two chief ray angles of light illumination andcollection;

FIG. 8 is a schematic illustration of an optical setup of a holographicinterferometer with no reference, according to some embodiments of thepresent invention;

FIG. 9 is a schematic illustration of an aperture plane, according tosome embodiments of the present invention;

FIG. 10 is a schematic illustration of an optical setup of a holographicinterferometer which is utilizes beam splitters to achieve fourdifferent effective wavenumbers, according to some embodiments of thepresent invention;

FIG. 11 is a schematic illustration of an optical setup of a holographicinterferometer for profiling a diffusive-reflecting object using ambientlight, according to some embodiments of the present invention;

FIG. 12 is a schematic illustration of an optical setup of a holographicinterferometer for profiling a diffusive-reflecting object usingpolarized light, according to some embodiments of the present invention;

FIG. 13 is a schematic illustration of an optical setup of a holographicinterferometer for profiling a distant diffusive-reflecting object usingpolarized light, according to some embodiments of the present invention;

FIG. 14 is a schematic illustration of an optical setup of a holographicinterferometer for profiling an object illuminated by light beams havingdifferent angles, according to some embodiments of the presentinvention;

FIG. 15 is a schematic illustration of an exemplary illumination patternconfiguration of an aperture plane of the holographic interferometer ofFIG. 14 , according to some embodiments of the present invention;

FIG. 16 is graph illustrating a simulation example of the dependence ofthe phase difference between the two beams, as well as the two reflectedbeams intensities, according to some embodiments of the presentinvention;

FIG. 17 is a schematic illustration of an optical setup of a holographicinterferometer for profiling an object illuminated by polarized lightbeams having different angles, according to some embodiments of thepresent invention; and

FIG. 18 is a schematic illustration of another optical setup of aholographic interferometer for profiling an object illuminated bypolarized light beams having different angles, according to someembodiments of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates toholographic imaging and, more particularly, but not exclusively, tomethods and systems of three dimensional measurements using holographicinterferometry.

According to some embodiments of the present invention, there isprovided a holographic interferometer having identical optical paths forthe object beam and the reference beam, to maximize uniformity of theinterference lines on the field of view. This is done by constructing areference image of a mirror, with essentially the same optical path thatis used for constructing the object image.

In many optical setups, it may be difficult to achieve uniforminterference line density across the entire field-of-view, as even whenthe optics is diffraction limited and the object is in focus, the phaseof an image point (relative to, for example, the image point at thecenter of field with a flat object field) may change rapidly when movingacross the field, especially away from the center of field (the opticalaxis). Typically for these setups, the reference image does not exhibitthis phase behavior, as it has a different optical setup than the objectimage, so the interference between the images produces an image in whichthe interference lines density changes across the field (even for a flatobject at focus), typically becomes denser the further from thefield-of-view center, and may even disappear when the interference linesbecome denser than the camera pixels. The twin optical paths holographicimaging setup solves this problem by ensuring that the phase differencebetween the object image and the reference image is essentiallydetermined by the object features and the planned angle between theimages.

According to some embodiments of the present invention, there isprovided a method of obtaining height measurement of a subject structurefrom an image produced by a holographic interferometer, by analyzing theimage using small blocks of pixels (super pixels) to find the phase andestimate the height for each one.

Common practice for finding the phase for every point in an interferencepattern image, is to use a Fourier transform over the whole image (or alarge portion of it, e.g. a region of interest), perform some operationsin the Fourier space (such as shifting the origin, digital filtering)and then inversing the Fourier transform back. The first difficulty isthat the operations in the Fourier space tend to smear the informationin the spatial space, hence introducing errors in the phase measurement.The second difficulty is that the Fourier transformation (and itsinverse) is computationally expensive, even when fast Fourier transform(FFT) algorithms are used. Using super pixels limits the spatialsmearing to the size of the super pixel, and is computationally cheaper.

According to some embodiments of the present invention, there isprovided a holographic interferometer using multiple angle split beamsfrom a single light source. This is done by separating the beamsarriving from different chief angles of collected light in the originalbeam (reflected from the object and/or reference mirror), usingapertures. Each aperture allows passage of light arriving from adifferent angles range so a different portion of the aperture plane isselected and transmitted. One or more apertures are used, located in anoptical path of one of the beams that are optionally split by a beamsplitter. Each of the apertures is located away from the axis of thesplit beam, optionally at a different angle than the other aperture(s).The exact position, shape and size of the aperture relative to the beamaxis controls the collected angles range of the beam that are used forthe path. Light transmitted through these different portions of theaperture planes creates the angle split beams. This idea may be used indifferent ways and with different optical setups to create aninterference pattern. For example, multiple angle split beams may beused to simulate multiple wavelengths and create multiple interferencepatterns, and/or may be used to interfere with each other to create theinterference pattern.

According to some embodiments, multiple interferometric images are takenusing a single wavelength, each of these images contains phaseinformation which corresponds to a different chief ray angle. The phasedependence on the height of a point depends on these angles, so theangle split beams may also be referred to as having ‘effectivewavelengths’. The effective wavelength is therefore controlled bychoosing the angle of the collected light that is used for the imageconstruction on the camera.

In many cases one interferometric image is not enough, as the dynamicrange of the height is limited by the wavelength used for the imaging. Acommon solution to this is to use several interferometric images, takenwith different wavelengths. Careful choice of the wavelengths enables toobtain a dynamic range larger than the wavelengths used for the imaging(this is known as unwrapping the phase, or as constructing a syntheticwavelength). Tunable laser sources with precisely controlled wavelengthsare needed in order to achieve a large dynamic range for heightmeasurement, and some tweaking in the wavelengths may be used tooptimize system performance. Using different effective wavelength from asingle wavelength light source thus removes the need for an expensive,multiple wavelength light source.

According to some embodiments, the holographic interferometer has noreference mirror, and the interference is between two images based ontwo different chief-ray angles of light collected from the object. Amonochromatic coherent light source may be used, or an external ambientillumination.

According to some embodiments, a diffusive-reflecting object may beprofiled using ambient light. Some of the ambient light reflected fromthe object is reflected in a specific direction and collected by anobjective lens. According to some embodiments, a diffusive-reflectingobject surface is illuminated with two different angle ranges, each witha different light polarization, optionally orthogonal. This increasesthe contrast of the interference lines.

According to some embodiments of the present invention, there isprovided a holographic interferometer where the illumination of theobject contains only some angles, so no filtration of the angle rangesis needed. The light source beam is split and the displacement of one orboth beams from the optical axis (in the aperture plane) is controlled,for example, by a mirror. The path difference between the beams iscompensated by a prism. This allows control of both the Z dynamic rangeand the interference lines density and angle on the camera.

Optionally, the beam is split by a polarizing beam splitter. This isadvantageous for measuring the amplitudes of the light reflected fromthe object, for example when the object consists of a transparent layerover a reflective surface.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium may be a tangible device that mayretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein may bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, may be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that may directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, may be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

Referring now to the drawings, FIG. 1 is a schematic illustration of anexemplary twin optical paths holographic imaging setup, according tosome embodiments of the present invention. The image of a referencemirror is constructed with essentially the same optical path that isused for constructing the object image—the optical paths of the twobeams are optically identical. A monochromatic coherent light source 101and a condenser lens 102 create an illumination field plane on a fieldstop 103. Beam splitters 104, 105, 106 and 107 split the light betweenthe object leg (a light beam which illuminates a subject structure 108)and the reference leg of the interferometer (a light beam whichilluminates a reference mirror 109), and then combine the reflectedimage on an imaging device, such as a camera 110.

Light source 101 may be, for example, light-emitting diode (LED),continuous wave (CW) lasers, and/or pulsed lasers. The subject structuremay be any type of object having height differences small enough forinterferometric measurement, such as integrated circuit (IC)semiconductor wafers, semiconductors, flat panel displays (FPDs) and/orprinted circuit boards (PCBs). The imaging device may be a light sensor,film, camera and/or any other type of light capturing device.

An angle between the object optical axis and the reference optical axiscreates a line interference pattern on camera 110, from which therelative phase between the object image and the reference image may bededuced. To get an interference pattern, the coherence length of theillumination needs to be longer than the optical path difference betweenthe object leg and the reference leg.

Optionally, this scheme may easily be extended to use multiplemonochromatic wavelengths by assigning a camera for each wavelength andusing dichroic beam splitters to send each wavelength image to itsassigned camera.

Optionally, a scheme using multiple wavelengths with a single camera isalso possible, by replacing mirrors 111, 112 with sets of dichroic beamsplitters, each set giving a different angle to the reference image ofeach wavelength.

Reference is now made to FIG. 2 , which is a flowchart schematicallyrepresenting a method of obtaining height measurement of a subjectstructure from an image produced by a holographic interferometer,according to some embodiments of the present invention.

First, as shown at 201, a digital image of an interference pattern,created by an imaging of a subject structure, is received from animaging device of a holographic interferometer.

The reference plane may be tilted in order to achieve sub-pixelresolution of the interferometric lines. For example, a tilt calculatedas: sin(axisangle)=1/(length of super pixel, in pixels), gets a uniformdistribution of sub-sampling.

Then, as shown at 202, a two-dimensional section of the image (alsoreferred to as ‘super pixel’ or analysis window) is selected. The superpixel is a group of pixels, for which the vertical axis information isobtained, such as height and/or thickness. The size of the super pixelmay be, for example, about the theoretical optical spot size, smallerthan ten times the theoretical optical spot size, smaller than a tenthof the size of the image and/or any other size.

For multi-wavelength optical setup using a single camera, the number ofsub-pixels in the super pixel is determined by the ability to extractthe phases reliably from the combined interference pattern. The size ofthe pixels on the subject structure depends on optical magnification ofthe image. Super pixel size (measured in number of pixels) is defined togive the individual phases for all wavelengths used. For example, threewavelengths arranged in one dimension need a minimum of 2*(number ofwavelengths)+1=7 pixels. Since the optical spot is symmetric, the superpixel in this case may be defined as 7×7 pixels. It is possible to use a2D arrangement, in which case a 5×5 super pixel may accommodate up to 4wavelengths, 7×7 may accommodate up to 6 wavelengths, etc. Working withsmaller super pixel may be used to increase the throughput, but it mayreduce phase accuracy (as fewer pixels are used to calculate thephases).

Reference is made to FIG. 3 , which is an exemplary super pixel from animage expected as a response to different feature z-height of thesubject structure, according to some embodiments of the presentinvention. Since the wavelengths do not interfere, the image in thecamera is the simple sum of the three images of each of the threewavelengths, as the camera in this setup is monochromatic.

Optionally, for multi-wavelength optical setup, separating between theimposed interference patterns of the different wavelengths may be doneby designing the reference beam angles so each wavelength interferencepattern has a different, substantially integer number of cycles insidethe super pixel. The reference reflection plane 109 use a small angle towafer for each wavelength, which may be chosen so there is a completenumber of cycles in the super pixel. The number of cycles may be one forthe first wavelength (λ₁), two for the second, etc. The angle isselected so that d sin θ₁=λ₁, d sin θ₂=λ₂, etc, while d is the size ofthe super pixel. This way, a simple Fourier analysis of the intensity ofthe pixels in the super pixel may give the phase of each wavelength, aswell as amplitude of the modulation per wavelength. An optical way toachieve this may be by injecting the references from different azimuthangles by separate, different optics than the image.

Then, as shown at 203, the interference pattern in the section isanalyzed to find the phase of the pattern, and to estimate height of arespective section of the subject structure.

The intensity of light field over a super pixel of n on m pixels indexedby x,y may be represented as:I _(x,y) =|E _(Object) +E _(Reference)|² =|Ae ^(iφ) Be ^(iθ) ^(x,y)|²=(A cos(φ)+B cos(θ_(x,y)))²+(A sin(φ)+B sin(θ_(x,y)))²=(A ² +B²+2AB(cos(θ_(x,y))cos(φ)+sin(φ)sin(θ_(x,y))))=A ² +B ²+2ABcos(θ_(x,y)−φ),where Ae^(iφ) represent the amplitude and phase of the object image, andBe^(iθ) ^(x,y) represent the amplitude and phase of the reference image.It is assumed that the amplitude and phase of the object image areconstant over the super pixel. This is approximately true for a superpixel of the size of the optical spot or smaller, and/or when the objecthas a flat surface with constant height at this area. The x,y dependencefor the phase of the reference beam arises from the interference lines(i.e. the angle between object and reference images).

Optionally, when n is an integer multiply of the interference linesspatial wavelength, the computation to extract the phase from the superpixel is very simple (FFT is not needed as only the phase of a singlefrequency is needed).

For example, when

${\theta_{x,y} = {x*2*\frac{pi}{n}}},$x=0, 1, . . . (n−1), the phase φ is found by using

$e^{i\;\varphi} = {\left( {\sum_{x,y}{e^{i\;\theta_{x,y}} \cdot I_{x,y}}} \right)/{{{\sum_{x,y}{e^{{ix}*2*\frac{pi}{n}} \cdot I_{x,y}}}}.}}$The values of θ_(x,y) may be determined in a calibration, e.g. where theobject has a flat surface, which is defined to be at zero height.

Optionally, when n is not an integer multiply of the interference linesspatial wavelength, the following example for an iterative procedure maybe used to extract the phase and amplitude of the interference lines ona super pixel.

-   -   1. Use calibration to determine θ_(x,y) and B (θ_(x,y) may be        determined as described above, B by using a non-reflecting        object such as a beam stop).    -   2. Estimate A using A²≈(Σ_(x,y)(I_(x,y)−B²))/(nm)    -   3. Solve the n*m linear, over-constrained equation set in [A        cos(φ)] and [A sin(φ)]: 2B cos(θ_(x,y))[A cos(φ)]+2B        sin(θ_(x,y))[A sin(φ)]−(I_(x,y)−A²−B²)=0    -   using the Moore-Penrose pseudoinverse of the coefficient matrix.    -   4. Extract A and φ from the solution.    -   5. When needed, repeat step 3 using the new estimation for A and        then repeat step 4. Repeat until A and φ converge.

Optionally, this algorithm may be extended to cases where severalinterference images are recorded on a single camera. In these cases,there are two variables for every interference image, and it isadvantageous to choose the (2D) spatial frequencies of the imagesinterference lines to minimize the noise amplification of the inversematrix.

Reference is now made to FIG. 4 , which is a schematic illustration ofan optical setup of a holographic interferometer for taking multipleinterferometric images (two for this example) with a single wavelength,according to some embodiments of the present invention. Each of theseimages contains phase information which corresponds to different chiefray angle and thus to a different angle split beam (or effectivewavelength), removing the need for using multiple true wavelengths andthus the need for an expansive, multiple wavelength light source.

The idea is to select for the different images different chief angles ofcollected light (reflected from the object). The phase dependence on theZ height of a point depends on these angles, hence the effectivewavelength (in regards to the dependence of the phase on Z) iscontrolled by choosing the angle of the collected light that is used forthe image construction on the camera.

Optionally, one or both aperture shift angles, position, shape and/orsize are dynamically controlled and/or changed in order to producemultiple images each having a different effective wavelength.Optionally, this setup may include only one camera, so image may betaken with a different effective wavelength at a different time.

Optionally, the focus and/or magnification of the optical system arechanged and/or dynamically controlled in order to produce multipledifferent images. Combining these images may increase the dynamic rangeof the height profiling or distance measurement. Optionally, the changesof aperture(s), focus and/or magnification are controlled by a computer.

In this example, the reference and object light paths are joined thesame way as in the example of FIG. 1 , but instead of a camera at theimaged field plane, a lens 401 is placed such that the imaged fieldplane is at the focal plane of the lens. Lens 401 images the apertureplane of the objective at a distance equal to its focal length. A beamsharer 402 is placed near the imaged aperture plane of the objective andsplits the beam in to two paths. In each path, an aperture 403 isplaced. Each aperture 403 is diagonally shifted (located away) from theaxis of the beam. The exact position, shape and size of aperture 403relative to the beam axis controls the collected angles range of thebeam that are used for the path. A re-image lens 404 in each path isused to re-image the object on each camera 405, using only the collectedangles range passed by aperture 403.

Reference is also made to FIG. 5 , which is a schematic illustration ofan aperture plane, according to some embodiments of the presentinvention. Circle 501 represents the objective acceptance cone of thebeam. Circle 502 represents a light cone which is normal to the surfaceof the aperture, i.e. an aperture not shifted from the axis of the beam,while circle 503 represent a cone of angle θ of the beam, i.e. anaperture shifted from the axis of the beam.

Reference is also made to FIG. 6 , which is a schematic illustration ofan alternative optical setup of a holographic interferometer for takingmultiple interferometric images with a single wavelength, according tosome embodiments of the present invention. Lens 601 (lens D) is placedat its focal distance before the conjugate field plane generated by thetube lens. Lens 601 moves the field plane to half the distance betweenitself and its focus, and generate an aperture plane at its focus. Abeam sharer (or a beam splitter) split the beam before the apertureplane, where the solid angle to use for each camera is selected. Lens602 (lens E) has half the focal length of lens 601, and it is placedwhere the aperture plane is at its focus, while the field plane is attwice that distance, so Lens 602 re-image the field plane on the camera(which is at a distance of twice its focal length).

Optionally, the optical setups described in FIG. 4 and FIG. 6 may bealtered to use multiple wavelengths with a single camera, by replacingthe mirrors with sets of dichroic beam splitters, as described for FIG.1 .

Reference is now made to FIG. 7 , which is a simplified example thatcompares between the optical path difference of two chief ray angles oflight illumination and collection, when the reflecting surface of anobject changes its height by Z.

The first angle is normal to the surface and the second angle is 0 tothe normal. For convenience of explanation, the initial surface positionis in focus, and the measured phase of both angles in the initial stateis zero.

For the first angle, the phase change is φ1=kL1=k*2Z, where k is thewave number in radians/unit length. For the second, the phase change isφ2=k(L_(2A)+L_(2B)), where L_(2A)=L_(2B) cos 2θ and L_(2B)=Z/cos θ soφ2=kZ (1+cos 2θ)/cos θ=k*2Z*cos θ. The phase change in the second caseis the same as for a normal angle case, but with an ‘effective wavenumber’ of k cos θ. So, when the effective wavelength corresponding tothe normal (first) angle is λ, the effective wavelength corresponding tothe second incident angle is λ/cos θ.

Optionally, the aperture position and diameter are controlled from acomputer to grant flexibility and/or calibration of the effective wavenumber. The aperture may include a circular hole and/or any other shape.

Optionally, additional beam splitters and/or beam sharers are added toutilize more than two interference images. This is useful to increasethe dynamic range of height measurement.

Reference is now made to FIG. 8 , which is a schematic illustration ofan optical setup of a holographic interferometer with no reference,according to some embodiments of the present invention. In this setup,the interference is between two angles of light collected from theobject.

A lens 801 images the aperture plane, beam sharer 802 splits the beamfor apertures 803. After re-imaging lenses 804, the two beams, each of adifferent angle, interfere on camera 805.

Reference is now made to FIG. 9 , which is a schematic illustration ofan aperture plane, according to some embodiments of the presentinvention. The large circle represents the objective acceptance cone ofthe beam, while the two smaller circles represent the cones of differentangles of the beam that are used for the interfering images.

The effective wave number in this case is k|cos(θ₁)−cos(θ₂)|, where θ₁and θ₂ are some effective angles to the normal in the object plane, andk is the single wavelength wave number. The corresponding effectivewavelength is λ/|cos(θ₁)−cos(θ₂)|.

It is advantageous to accurately align the two interfering images on thecamera, to reduce spatial smearing of the combined image.

Optionally, additional beam splitters and/or beam shares are added toproduce additional interference images, with different effective wavenumbers. Reference is now made to FIG. 10 , which is a schematicillustration of an optical setup of a holographic interferometer whichis utilizes beam splitters to achieve four different effectivewavenumbers, according to some embodiments of the present invention.This scheme may be used to increase the dynamic range of heightmeasurement by a power of 4.

Optionally, the object illumination may be external to the shown system.The coherence length still needs to be longer than the optical pathdifference between the two interfering images, and in addition,coherence is needed between the different angles of collection.

Optionally, a spectral filter is added to the optical system. Thespectral filter increases the coherence length, as well as reducessmearing of the interference lines. The optimization of this filter willdepend on the application and on the external illumination.

Reference is now made to FIG. 11 , which is a schematic illustration ofan optical setup of a holographic interferometer for profiling adiffusive-reflecting object using ambient light, according to someembodiments of the present invention.

A diffusive-reflecting object reflects the light such that a rayincident on the surface of the object is scattered at many angles ratherthan at just one angle. Therefore, some of the ambient light reflectedfrom the object is reflected in a specific direction and collected byobjective lens 1101. Lens 1102 creates two conjugate aperture planesafter beam splitter 1103, which splits the light beam into two splitbeams. Each of the split beams is filtered by one of aperture stops 1104to select a different solid angle. Optionally, the optical path of thetwo beams is equal. Lenses 1105 and 1106 are used to relay the conjugatefield plane to the camera. The beams are combined by beam splitter 1107and interfere, at an angle, on a camera 1108. The optical pathdifference between the two beams depends on the distance of the objectfrom the system focal plane, as well as on the wavelength and solidangles between the optical axis and the object. By analyzing the phaseof the interference pattern on the camera, the distance to the objectcan be calculated, and the object may be profiled.

Optionally, calibrations may be used to correct for optical aberrationseffects. Calibrated parameters may include, for example, focal planelocation and/or Z-phase dependence per field location on camera.

Optionally, a spectral filter is used to increase the coherence of thecollected light, so that the two images on the camera may generate theinterference lines over a larger range of path differences.

Optionally, one or more optical components, are changed and/ordynamically controlled (for example moved, tilted and/or changed in anyway) to control the focus plane of the system. This may be advantageouswhen a broad optical spectrum bandwidth is needed (hence the coherencelength is short) or for handling a large Z range (where the pathdifference may be larger than the coherence length). Optionally, thechanges in the optical components are controlled by a computer.

In some cases, when the object's surface is diffusive-reflecting, thecontrast of the interference lines may degrade and/or the relationbetween the surface height and interference lines phase may depend onthe scattering characteristics of the surface.

Reference is now made to FIG. 12 , which is a schematic illustration ofan optical setup of a holographic interferometer for profiling adiffusive-reflecting object using polarized light, according to someembodiments of the present invention.

The object surface is illuminated with two different angle ranges, eachwith a different light polarization. Optionally, the two polarizationsare orthogonal. In the light collection part of the setup, the reflectedtwo polarizations are separated, and each is filtered by apertures sothat the angle range that corresponds to a specular reflection (for thatpolarization) is passed on. This setup can thus handle specular,partially diffusive-reflecting or highly diffusive-reflecting surfaces.

Lenses 1201 and 1202 image the light source to the conjugate apertureplanes after polarizing beam splitter 1203. Linear polarizer 1204 isoriented at 45 degrees to the figure plane to ensure coherence betweenthe polarizations. At each aperture plane the two split beams arefiltered by each of apertures 1205 and 1206 to select a different solidangle. The beams are combined by polarizing beam splitter 1207 andcontinue to illuminate the object. On collection, the two polarizationsare separated by polarizing beam splitter 1208. Each of the split beamsis filtered at a conjugate aperture plane by one of apertures 1209 and1210 to select a different solid angle. The polarization of one of thebeams is rotated by half-wavelength (λ/2) waveplate 1211 while the otherpasses through a window 1212, so that the two beams interfere on camera1214 after being combined by beam splitter 1213.

Optionally, the angle ranges of the two polarizations paths in thecollection part of the optical setup are identical. This can beadvantageous on curved surfaces, which change the angle of thespecular-reflected light. It may also be advantageous ondiffusive-reflecting and out of focus surfaces with large heightdifferences inside the optical point spread function area, as it ensuresthat both interfering beams image essentially the same object area on acamera field point, even when the imaged surface is out of focus. Inthis case the effective wavelength is determined by the light wavelengthand the angle range selection on the illumination part. Optionally, anyother arrangement of angle range selection may be used, as isadvantageous to a specific application.

Optionally, the dynamic range of the height measurement is be increasedby adding one or more wavelengths. The additional wavelengthsinterference lines may be imaged on separate cameras, or be combined totwo or more interference patterns on one camera.

Optionally, this setup is combined with the previous setup schemes tooptimize the system design for a specific application.

Reference is now made to FIG. 13 , which is a schematic illustration ofan optical setup of a holographic interferometer for profiling a distantdiffusive-reflecting object using polarized light, according to someembodiments of the present invention. The illumination utilizesdifferent illumination angle ranges for the two polarizations.Optionally, the angle ranges of the two polarizations paths in the lightcollection part of the optical setup are identical.

The optical schemes which are described above with no reference arebased on filtration of the angle ranges. It is also possible to designsimilar optical schemes, in which instead of filtering out the unwantedangles, the illumination contains only the needed angles in the firstplace.

Reference is now made to FIG. 14 , which is a schematic illustration ofan optical setup of a holographic interferometer for profiling an objectilluminated by light beams having different angles, according to someembodiments of the present invention. The light source may be, forexample, a fiber or a fiber bundle that passes light from a lasersource.

The optics duplicates the light source beam and control the displacementbetween the beams, while enable to keep the path difference between thebeams to essentially zero, if needed. By controlling the displacement ofboth beams from the optical axis (in the aperture plane), it is possibleto control both the Z dynamic range and the interference lines densityand angle on the camera.

Beam splitter 1401 split the light to two beams, where beam splitter1402 combines them back. Mirror 1403 is displaced (along an axis shownby an arrow) to change the distance of one beam from the originaloptical axis. This creates an angle of incident of the beam on theobject. Different displacements of the one beam from the optical axis,translate to different incident illumination angles on the object.Displacement of prism 1404 optionally compensates for the path lengthdifference between the beams.

Reference is now made to FIG. 15 , which is a schematic illustration ofan exemplary illumination pattern configuration of an aperture plane ofthe holographic interferometer of FIG. 14 , according to someembodiments of the present invention.

The dependence of the interference lines phase φ on Z is given byφ=k*2Z*(cos θ₁−cos θ₂), where k is the angular wave number, θ₁ and θ₂are the illumination angles to the optical axis. The density of theinterference lines on the camera is given by k|sin θ₁−sin θ₂|/M where Mis the optical magnification.

For some applications it may be advantageous to measure the amplitudesof the light reflected from the object. This may be true, for example,when the object consists of a transparent layer over a reflectivesurface. In this case, the transparent layer changes the phase of thereflected light, so it may be difficult to measure the Z of thereflective surface, and/or the Z of the transparent layer top surface,separately.

Reference is now made to FIG. 16 , which is graph illustrating asimulation example of the dependence of the phase difference between thetwo beams, as well as the two reflected beams intensities, according tosome embodiments of the present invention.

Measuring the intensities of the reflected light may give information onthe thickness of the transparent layer, and be used to “correct” thephase, so that a correct Z of the transparent layer top may becalculated. It is possible to deduce the beams intensities from themeasured “DC” and modulation intensities of the interference lines onthe camera, using:A _(H) +A _(L)=√{square root over (I _(DC) +I _(modulation))}A _(H) −A _(L)=√{square root over (I _(DC) −I _(modulation))}

where H and L subscripts designate high and low amplitudes. Withoutfurther knowledge, there may be ambiguity in the correspondence betweenthe amplitudes and the illumination angles (i.e. knowing which amplitudebelongs to which angle).

This ambiguity may be resolved by polarization. Reference is now made toFIG. 17 , which is a schematic illustration of an optical setup of aholographic interferometer for profiling an object illuminated bypolarized light beams having different angles, according to someembodiments of the present invention. Light from a linearly polarizedlight source passes through a λ/2 waveplate 1701 and is split by apolarizing beam splitter 1702. After the change of angles, the beams arecombined by a polarizing beam splitter 1703. The two illuminating beamshave different, optionally orthogonal polarizations. The holographicinterferometer of FIG. 17 uses linear polarizations as an example, butother polarizations can be used. The combined beams reflected from theobject are split to two cameras 1704 and 1705 by beam splitter 1706. Thelinear polarizers 1707 and 1708 have each their axis at a differentangle to the beams polarization directions. By using two differentpolarizer axis angles for the two cameras, it is possible to identifywhich amplitude correspond to which polarization (hence to whichillumination angle).

Reference is now made to FIG. 18 , which is a schematic illustration ofanother optical setup of a holographic interferometer for profiling anobject illuminated by polarized light beams having different angles,according to some embodiments of the present invention. In this setup itis possible to manipulate the density of the interference linesindependently from the illumination angles and control the relativeangle of incident of the two beams on the camera. This is possible by asecond set of polarizing beam splitters, prisms and mirrors, which isplaced at the collection part of the optical setup, and enable controlof the collection angles of the two polarizations, independently of theillumination angles.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

It is expected that during the life of a patent maturing from thisapplication many relevant interferometers will be developed and thescope of the term interferometry is intended to include all such newtechnologies a priori.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”. This termencompasses the terms “consisting of” and “consisting essentially of”.

The phrase “consisting essentially of” means that the composition ormethod may include additional ingredients and/or steps, but only if theadditional ingredients and/or steps do not materially alter the basicand novel characteristics of the claimed composition or method.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration”. Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

The word “optionally” is used herein to mean “is provided in someembodiments and not provided in other embodiments”. Any particularembodiment of the invention may include a plurality of “optional”features unless such features conflict.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

What is claimed is:
 1. A holographic interferometer having no referencemirror, comprising: at least one imaging device capturing aninterference pattern created by at least two light beams collected froman illuminated subject structure; and at least two apertures, eachlocated along an optical path of one light beam of said at least twolight beams; wherein said at least two apertures are each differentlylocated away from an optical axis of a respective one of said at leasttwo light beams, thus transmitting a substantially different subset ofsaid at least two light beams of different collected angles range havingdifferent effective wave number; and wherein each of said at least twolight beams is arriving into said at least one imaging device at adifferent angle with respect to said at least one imaging device, tocreate said interference pattern.
 2. The holographic interferometer ofclaim 1, further comprising: at least one beam splitter which splits anoriginal light beam into said at least two light beams.
 3. Theholographic interferometer of claim 2, wherein said original beam is abeam reflected from a subject structure.
 4. The holographicinterferometer of claim 3, further comprising monochromatic coherentlight source illuminating said subject structure.
 5. The holographicinterferometer of claim 3, wherein said subject structure is illuminatedby ambient light.
 6. The holographic interferometer of claim 1, whereinsaid at least two light beams are reflected from a subject structure. 7.The holographic interferometer of claim 1, wherein said interferencepattern is used for obtaining height measurement of a subject structure.8. The holographic interferometer of claim 3, further comprising: a lenslocated along an optical path of said original light beam and generatesan aperture plane; wherein said beam splitter is located before saidaperture plane along said optical path of said original light beam, thussplitting said aperture plane.
 9. The holographic interferometer ofclaim 8, wherein said at least two apertures are located at said splitaperture plane.
 10. A method of setting a holographic interferometerhaving no reference mirror, comprising: positioning at least one imagingdevice for capturing an interference pattern created by at least twolight beams collected from an illuminated subject structure; andpositioning at least two apertures, each along an optical path of onelight beam of said at least two light beams; wherein said at least twoapertures are each differently located away from an optical axis of arespective one of said at least two light beams, thus transmitting asubstantially different subset of said respective one of said at leasttwo light beams, each said subset includes light collected at differentangles having different effective wave number; and wherein said each ofsaid at least two light beams interferes on said at least one imagingdevice at a different angle.
 11. A holographic interferometer,comprising: at least one beam splitter which splits an original lightbeam into at least two light beams; and at least one imaging devicecapturing an interference pattern created by said at least two lightbeams after reflection from an object; wherein said at least two lightbeams have different angle of incident on said object corresponding todifferent effective wave number.
 12. The holographic interferometer ofclaim 11, further comprising: at least one mirror which changes thedistance of at least one light beam of said at least two light beamsfrom an original optical axis of said original light beam to create saiddifferent angle of incident.
 13. The holographic interferometer of claim11, wherein said at least one beam splitter includes at least onepolarized beam splitter.
 14. The holographic interferometer of claim 12,wherein said at least one imaging device includes at least two imagingdevices, each capturing an interference pattern created by light havingdifferent polarization angle.